Lipoprotein lipase (LPL) is rate limiting for the hydrolysis of triglycerides in circulating triglyceride-rich lipoproteins. LPL also contributes to the delivery of lipid fuels to tissues where they are either stored (adipose tissue) or oxidized (muscle). LPL regulation in adipose tissue and muscle is often divergent, and alterations in LPL and its regulation by hormones and nutrients are commonly seen in obesity. Therefore, additional insights into the relative contribution of LPL- vs hormone sensitive lipase (HSL)-mediated provision of free fatty acids (FFA) in meeting the oxidative fuel needs of muscle are needed. Such information may ultimately be helpful in targeting gene-based therapies to disturbances of body weight regulation. The overall working hypothesis is that increases in fat oxidation will result in insulin resistance, a metabolic environment which may prevent or deter weight gain. Studies outlined in this grant will exclusively utilize transgenic and control mice with muscle or adipose tissue-specific overexpression of LPL or HSL, following exposure to either a high carbohydrate or high fat diet. Investigations will focus on the following specific hypotheses that: 1. overexpression of LPL in muscle will enhance lipid oxidation, reduce insulin sensitivity, and reduce adipose tissue mass; and, that overexpression of LPL in adipose tissue will decrease lipid oxidation, enhance insulin sensitivity, and increase adipose tissue mass; 2. overexpression of HSL in adipose tissue and/or muscle will enhance lipid oxidation, decrease insulin sensitivity, and decrease adipose tissue mass; 3. overexpression of LPL in muscle, and HSL in muscle or adipose tissue of diet sensitive C57BL/6J mice will result in a less pronounced phenotype than in FVB mice; and, that overexpression of LPL in adipose tissue of C57BL/6J mice will produce a more pronounced phenotype than in FVB mice; and 4. the hybrid offspring from matings of muscle-specific LPL, muscle- specific HSL, or adipose tissue-specific HSL overexpressing FVB mice to other transgenic mouse models of obesity (i.e. the adipose tissue specific GLUT4 overexpressing mouse (GLUT4), and the brown adipose tissue deficient (UCP-DTA) mouse), will have delayed development and a less pronounced degree of obesity. While testing these hypotheses, the potential impact of tissue-specific overexpression of the lipase transgenes on energy balance will be carefully monitored, and levels of the ob gene product mRNA in adipose tissue and ob protein in serum quantified to test the putative role of this peptide in body weight regulation. Overall, these studies should provide important and new insights into tissue-specific lipid fuel metabolism which may result in targeted gene-based therapies to alter nutrient fuel partitioning, and ultimately body weight.